Digital cameras utilizing high-resolution electronic imaging sensors (so-called "megapixel" cameras) require high resolution optics. For the consumer market, it is important that the lenses can be produced in high volume inexpensively. With many sensors, a mechanical shutter and/or a variable aperture are necessary or desirable to optimize the imaging performance of the sensor.
A variable aperture is generally desirable to enable the camera to take acceptable pictures in a wide range of lighting conditions. Where the environment is darker, the aperture must be fully open to allow sufficient light to illuminate the sensor at an optimum level. In a brighter environment the aperture must be reduced to limit the intensity of light reaching the sensor to the optimum level and prevent saturation of the sensor. Also, the depth of field is increased when the aperture is reduced, allowing objects to be in good focus over a greater range of distances.
For some types of sensors, generally known as "frame-transfer sensors", the active area of the sensor must be shielded from illumination completely during image read-out from the sensor to obtain clean data from the sensor. Typically, this is accomplished using a conventional mechanical shutter of the sort used in film cameras.
A single mechanical device can perform both shutter and variable aperture functions. Such a dual-function device is advantageous, since it requires less space than separate, independent, shutter and variable aperture devices, making the lens assembly more compact and less expensive. For proper functioning of the variable aperture, the dual-function device must be located at the aperture stop (also known as the iris position) of the lens design, at the conventional location where an independent variable aperture would be located.
In the prior art, high resolution lenses have generally been made up of several individual lens elements in order to balance the inherent optical aberrations. These lenses that require a large number of elements tend to be relatively large, heavy and expensive to manufacture. The cost of these lenses increases with the number of elements and the resulting increased costs in assembling and mounting them in a lens cell. Prior lenses are generally designed using all spherical surfaces or using at least some aspheric elements in which one or both surfaces are non-spherical.
Where all elements have spherical surfaces, generally a high number of lens elements is required, making the lens long and heavy and expensive to produce. A compact lens is required for such devices as pocket size consumer cameras, cell phones and personal digital assistants.
Aspheric lenses have some optical advantages, but cannot be easily produced by traditional grinding and polishing techniques. Aspheric elements are typically produced by molding plastics or low melt temperature glasses. While molded plastic elements are inexpensive to produce, the level of precision of the lenses is not sufficient for high-resolution cameras. In addition, the optical properties of most plastic materials change with changes in temperature and humidity. While it is possible to make glass aspheric lens elements, manufacturing to the required accuracy is difficult and expensive when compared to manufacture of spherical lens elements.
Prior lens designs generally have separate variable apertures and shutters, increasing the length of the lens assembly. Even where both these functions are combined in one device, that device must be positioned between lens elements because the aperture stops of conventional designs are located between lens elements.
Having the aperture stop between lens elements, as in the Double Gaussian designs, is believed to make correction of aberrations easier. Typical of such lens designs is that described by Fugii in U.S. Pat. No. 4,212,517, where the aperture stop is located between the third and fourth elements. This provides a degree of lens symmetry about the apertures stop, resulting in reduction in off-axis aberrations such as coma and distortion. It is generally believed that achieving good aberration correction without this symmetrical arrangement of lens elements would be difficult. However, it is difficult and expensive to integrate a variable aperture/shutter device with this type of optical design since it is difficult to keep the lens elements precisely with the aperture device located between the elements.
Defuans, in U.S. Pat. No. 4,525,039, describes a lens design with the aperture stop in front of the first element. That design requires that the first element must be plano-convex, with the plano surface facing the aperture. However, that deign has a maximum relative aperture of f/4, too slow for use with cameras to be used at relatively low light levels. That design further requires seven elements, making it excessively long, heavy and expensive to products for use in compact cameras, especially in compact digital cameras.
Therefore, there is a continuing need for improved lenses that have excellent low-light performance and are compact, short, light weight and inexpensive to produce while using conventional, well-proven manufacturing methods.